1. Field of Invention
The present invention relates generally to a method of fabricating a semiconductor. More particularly, the present invention relates to a method and structure of fabricating a wafer by baking.
2. Description of the Related Art
A polyimide has good insulating and high adhesive abilities. A polyimide is often used for the process of fabricating semiconductors. A polyimide is used as a protective layer, a buffer layer or a dielectric layer. In semiconductor fabrication, when forming a polyimide layer, a solvent is usually added into the precursor of the polyimide to form a solution. The solution is then applied on a substrate. When polyimide material is exposed to light, it will cause cross linking and patterning. Aside from the above-mentioned functions, forming a polyimide layer on the substrate also has a function of forming a patterned photoresist layer. Thus, the steps of the process can be reduced, and semiconductor fabrication can be applied more extensively, for example in forming a fuse wire pattern.
The polyimide layer is in a liquid state when the polyimide is applied on the surface of the substrate. The wafer will be subjected to a soft baking process to evaporate all the solvent in polyimide. The wafer will be then subjected to exposure and development processes sequentially.
The wafer baking method uses a hot plate to supply heat to the wafer in order to evaporate all the solvent in polyimide in the wafer. In FIG. 1A, a hot plate 100 is provided, and a plurality of pin holes 102 are formed on the hot plate 100. The positions of the pin holes are located to form a plurality of concentric circles, and these circles are formed with different sizes to correspond to various sizes of the wafers. Spacers 104 that can be made of a metal or a ceramic material are located in the positions of the pin holes 102 that correspond to the size of the wafer. The wafer coated with a polyimide layer 108 is located on the spacers 104. The wafer 106 is then subjected to a baking process in order to slightly solidify the polyimide layer 108.
FIG. 1B illustrates the contact position of the spacers 104 and the wafer 106. The wafer 106 is covered with the polyimide layer 108, and the edge of the back surface of the wafer 106 is in contact with a portion of the spacer""s surface. That portion of the surface of the spacer 104 is exposed on the outside edge of the wafer 106. The baking method of the wafer mentioned above has the following disadvantages. If the material of the spacer is metal, the adhesive ability of the polyimide is very high, and the conductive coefficient of the spacers is high. The spacers will conduct heat easily and have a high temperature during the baking process. The wafer will heat up easily and cause the polyimide layer to melt and flow from the wafer surface to the edge. After some days of the operation, the accumulation will form polyimide residue on the spacers and the wafer edge. This polyimide residue adhered on the wafer edge can cause problems to the equipment and the wafer. For example, the polyimide residue on the wafer edge can cause a wafer notch search to an error and cause the barcode position to be shifted, a vacuum error problem on the stepper wafer chuck, or an equipment alignment error. If the spacer is made of ceramic material, the ceramic spacer also has the problems of the metal spacer mentioned above, and the ceramic spacer is very brittle as well. Once the spacer breaks, it will be adhered on the backside of the wafer.
After some days and continuous operation, the polyimide residue will pile up in the spacers. So all the spacers will need to be changed, approximately once a week, and 6 spacers are changed each time. Therefore the utilization rate of the equipment is decreased, and the cost of the equipment maintenance is high.
It is an object of the present invention to provide a method and structure of baking a wafer to avoid the formation of the polyimide residue on the wafer edge and improve the yield of the wafer. The present invention aims to avoid an uneven heat diffusion during a baking process of forming a polyimide layer. The polyimide layer then can be formed evenly on the wafer and the utilization rate can be improved. Therefore the cost of the process can be reduced.
The present invention provides a method and structure for baking a wafer in which the wafer that is covered with a material layer is placed on a hot plate to perform the baking process. The positions of the spacers that support the wafers are moved from the edge of the wafer to the inner side of the wafer. The spacers also have small contact areas to prevent the edge of the wafer from coming into contact with the spacers. A plurality of pin holes are formed on the hot plate, and the spacers are located in the pin holes. The number of spacers required depends on how many spacers are needed to stabilize the wafer on the hot plate. The spacers are made of a non-metal material that has high thermal resistance and low conductivity, so the spacers are able to come into contact with the backside of the wafer.
It is another object of the present invention to prevent the surfaces of the spacers from coming into contact with the edge of the wafer. The positions of the spacers that support the wafer are moved from the edge to the inner side of the wafer to avoid forming residue on the spacers that will adhere on the edge of the wafer if they come into contact with each other. The spacers are made of a non-metal material that has high thermal resistance and low conductivity, so the temperature of the contact region of the spacers and the wafer will not become too high. A problem of the conventional method is that a portion of the wafer has a high temperature and will cause the temperature to distribute unevenly on the wafer. The number of spacers used in the present invention is less than the conventional method. The spacers also will not be adhered to some residue. Therefore the spacers will be maintained in good condition and the utilization rate of the spacers will also increase. The cost of changing the spacers will then be reduced extremely.
The spacers of the present invention can be made into a conical structure. The tip of the conical spacer is in contact with the backside of the wafer. The contact area is thus reduced to avoid the uneven distribution of heat on the wafer during the baking process.